1. Field of the Invention
The present invention relates to a wavelength converter capable of obtaining a stable high output visible light laser beam by combining a fiber laser and a wavelength conversion element, and a two-dimensional (2D) image display device using this wavelength converter as a light source.
2. Description of the Background Art
A visible light source capable of emitting a highly monochromatic W-class high output is being required to realize large-size displays, high-luminance displays, etc. High-output red semiconductor lasers used in DVD recorders and the like can be utilized as small-size light sources having high productivity for red light out of three primary colors of red, green and blue. For green and blue light sources, however, realization by semiconductor lasers and the like is difficult and small-size light sources having high productivity are still asked for. Above all, it is highly difficult to realize green light sources since there is no material suitably usable for semiconductor lasers to obtain green output beams.
Wavelength converters as combinations of fiber lasers and wavelength conversion elements are realized as low-output visible light sources for green and blue lights. Small-size green and blue light sources using a semiconductor laser as a light source for excitation light for exciting the fiber laser and using a nonlinear optical crystal as the wavelength conversion element are well-known.
However, several problems need to be solved in order to obtain W-class high-output green and blue beams from such wavelength conversion elements. For example, in the case of obtaining a green output beam using the construction of a conventional wavelength converter, the wavelength converter needs to include a fiber laser for outputting a fundamental wave, a wavelength conversion element for converting the fundamental wave into a green laser beam and a lens for condensing an output of the fundamental wave to an end surface of the wavelength conversion element.
Here, a basic laser operation of this fiber laser is described. First, an excitation light from an excitation laser light source is incident on one end of a fiber. The incident excitation light is absorbed by a laser-active material contained in the fiber, whereby a seed light of the fundamental wave is generated in the fiber. This seed light of the fundamental wave reciprocates by being reflected many times in a resonator using a fiber grating formed in the fiber and a fiber grating of another fiber as a pair of reflection mirrors. Simultaneously, the seed light is amplified by a gain by the laser-active material contained in the fiber to increase its light intensity and to have a wavelength selected, thereby reaching a laser oscillation. It should be noted that the two fibers are connected by a connecting portion and the laser light source is current-driven by a laser current source for excitation.
Next, a basic operation of the wavelength converter is described. The fundamental wave is outputted by the fiber laser as described above to be incident on the wavelength conversion element via the lens. This fundamental wave from the fiber laser is converted into a harmonic by the nonlinear optical effect of the wavelength conversion element. The converted harmonic is partly reflected by a beam splitter, but the other part having passed through the beam splitter becomes a green laser beam as an output beam of the wavelength converter.
The harmonic partly reflected by the beam splitter is converted into an electrical signal to be used for the monitoring of the output beam from the wavelength converter after being received by a light receiving element. An output controller regulates a drive current of the laser light source by means of a laser current source for excitation so that the intensity of the converted signal becomes an intensity to give a desired output in the wavelength converter. Then, the intensity of the excitation light from the laser light source is regulated and the output intensity of the fundamental wave from the fiber laser is regulated, with the result that the output intensity of the wavelength converter is regulated. In this way, a so-called automatic power control (hereinafter, abbreviated as “APC”), in which the output intensity of the wavelength converter is kept constant, is stably performed.
Green high-output laser beams of several hundreds mW can be obtained by the above construction, but it is difficult to obtain W-class green high-output laser beams. Specifically, the outputs of the fundamental wave and excitation light of the fiber laser need to be increased in order to increase the light output of the wavelength converter. On the other hand, it is known that a natural emission called an ASE (Amplified Spontaneous Emission) occurs in a fiber laser light source doped with Yb as a laser-active material and is irradiated to an excitation laser light source (so-called return light) to cause the deterioration of the excitation laser light source. There is another problem that this ASE is generated by unintended light reflection outside a laser resonator to destroy a nonlinear optical crystal for generating a second harmonic. In order to prevent the former problem of the deterioration of the excitation laser, there have been conventionally proposed a method using a dichroic mirror (Japanese Unexamined Patent Publication No. 2004-165396), a method for connecting fibers at an angle (Japanese Unexamined Patent Publication No. 2005-70608), a devised construction of a fiber (Japanese Unexamined Patent Publication No. 2005-159142), the use of a reflection amount regulator for regulating the light quantity of a return light (Japanese Unexamined Patent Publication No. 2003-318480) and the like.
However, there is a high possibility that the wavelength of the ASE generated becomes 1040 nm to 1080 nm close to that (e.g. 915 nm, 975 nm) of an excitation laser in the case of trying to obtain the wavelength (1030 nm to 1100 nm) of a fundamental wave for a laser display. In such a case, excitation efficiency decreases and it is difficult to effectively prevent the ASE from returning to the excitation laser light source through the wavelength selection by a dielectric filter or the like. In the method for connecting the fibers oblique to each other, there has been a problem of deteriorating the connecting portion by the generated ASE.
The wavelength of the ASE generated in the case of trying to obtain the wavelength (1100 nm to 1180 nm) of a fundamental wave for a laser light source for medical use is relatively distant from that of an excitation laser. Thus, the return of the ASE can be hindered through the wavelength selection by a dielectric filter or the like in such a case, but it is expected to more easily build a construction for hindering the return of the ASE.